1. Field of the Invention
The present invention relates to a fingerprint matching method and apparatus. In particular, the present invention relates to a fingerprint matching method and apparatus that can achieve high accuracy in matching fingerprints having fewer minutiae, such as endpoints and bifurcations in fingerprint ridges.
2. Description of the Related Art
A commonly-used conventional method of matching fingerprints has been to match fingerprint minutiae by focusing on endpoints and bifurcations in fingerprint ridges (endpoints and bifurcations are hereinafter referred collectively to “minutiae”), as is disclosed in Japanese Patent No. 2659046.
The above-mentioned conventional method, however, has several drawbacks, as follows.
Since this method uses minutiae alone as features of fingerprints, it can guarantee high matching accuracy only when there are a sufficient number of minutiae. However, it cannot achieve high matching accuracy when minutiae are inadequate in number, which case is often encountered with fragmental latent fingerprints.
In order to solve the problem of low matching accuracy experienced when minutiae are inadequate in number, an alternative method has been proposed that extracts non-minutia data and uses both minutiae and non-minutia data for matching fingerprints, as is disclosed in Japanese Patent No. 002730179. In this method, when there are no minutiae in a small zone of a pre-determined size, the central coordinates of that zone and the ridge direction in the same zone are registered as non-minutia data. This method, however, does not use the degree of stability of skeletons in relation to the size of the zone having no minutiae.
Furthermore, the method disclosed in the above-described patent handles a non-minutia or minutia as one checkpoint indiscriminately, and verifies the possibility that such non-minutia or minutia contained in a fingerprint on the search side may form a pair with the corresponding non-minutia or minutia in a fingerprint on the registration side. This approach inevitably places a limit on the effectiveness of this method. For example, if one fingerprint has a minutia at a particular position and the other fingerprint has a non-minutia at the corresponding position, the minutia and non-minutia can be effectively used in a subtraction for matching scores (penalization).
On the other hand, if both a fingerprint on the search side and a fingerprint on the registration side have non-minutia data, their scores will be subjected to an addition process. This way, the resultant degree of separation of the non-mate fingerprints based on these scores is not large. The reason for this is that many non-minutia data are extracted from the non-mate fingerprints, making it difficult to separate the mate fingerprint from the non-mate fingerprints. One such example will be described below with reference to a diagram attached hereto.
FIGS. 7A and 7B shows a fingerprint on the search side. Of these, FIG. 7A shows a latent fingerprint and FIG. 7B a sample gray scale image of a mate fingerprint. FIGS. 8A and 8B show skeleton data for FIGS. 7A and 7B, respectively.
In FIGS. 8A and 8B, less reliable areas are enclosed in shaded rectangles.
In relation to the skeleton data in FIGS. 8A and 8B, FIGS. 9A and 9B represent non-minutia data, N1S, N2S, . . . , N7S, in the search-side fingerprint and non-minutia data, N1F, N2F, . . . , N7F, in the mate fingerprint, respectively, all of which were extracted using the method described in Japanese Patent No. 002730179. In a collation with this mate fingerprint, all the seven non-minutia data shown in this diagram match and thus a score of seven points is assigned.
FIGS. 10A and 10B show non-minutia data and minutiae when the fingerprint on the registration side is a non-mate fingerprint. Extracted from this example are six non-minutia data, N1F, N2F, N3F, N5F, N6F, and N7F, as well as one minutia, M4F. In a collation with this non-mate fingerprint, a score of 6 points is assigned because six non-minutia data match, while 1 point is subtracted from this score as a penalty because the non-minutia, N4S, on the search side does not match. The resultant score is 5 points.
The 7-point score for the mate fingerprint and the 5-point score for the non-mate fingerprint do not represent a sufficient degree of separation, demonstrating the need for an improved method with higher matching accuracy.